So did the introduction of single leg pricing just involve abolishing some through fares rather than replacing them with single leg equivalents?
It involved:
1. Reducing the Super Off Peak Single to half the Super Off Peak Return plus about 10% (later another 5-10% or so was very sneakily stuck on top).
2. Removing the Off Peak Single and Return, which on LNER were shoulder-peak "business saver" type tickets, as indeed they are on GWR but not the WCML.
The aspect mentioned, though, is that you can't usefully have asymmetric restrictions or fares* if you do single fare pricing, though which set of restrictions apply in a non-obvious "Network Rule" type situation is a bit moot, one set will be an advantage to some people and the other set to other people, it's hard to say one's right and one's wrong.
* Example, if you're talking about a South East commuter journey you may want to apply looser restrictions to a return out of London than a return into it, i.e. you want your inbound commuters to buy an Anytime Day Return if returning between 4 and 7pm, say, but you want an evening daytripper from London to be able to use an off peak for an outward at those times or they simply won't bother.
I'm still struggling to see how single leg pricing for off peak returns paved the way for a scheme that involves abolishing off peak fares completely.
It didn't really other than that there were two steps in LNER's plans. The one possibility I can think of is that for most people the outward time of a journey is relatively fixed*, flexibility tends to be needed on the return, so the effect of it may have been to increase Advance sales as people would tend to do Advance out, flexible back.
* One thing the 70 minute flex does fit very well is delays caused by other modes causing the missing of one train, e.g. a late or cancelled bus to the station.
(Or maybe my assumption that anytime singles were previously half the return is wrong)
They mostly are and were, or at least thereabouts. They only tended not to be on commuter journeys, to discourage the purchase of a single in and a Railcard discounted single back (which because they now largely are half the returns is better value than a flexi season for a good many South East mid-length journeys).